Project Summary Abstract TheMre11-Rad50-Nbs1complexisapleiotropicsensorofDNAdoublestrandbreaksthatpromotesATM signaling,cellcyclecheckpointactivation,andDNArepair.WehaverecentlyshownthatMre11isanessential componentofthetumorsuppressiveDNAdamageresponse(DDR)inamurinemodelofbreastcancer.Mre11 pathwaydeficiencyhasalsobeenreportedinasignificantfractionofhumantriplenegative (estrogen/progesteronereceptornegative,HER2non-amplified)breastcancers(TNBC),whichare characterizedbyrampantchromosomalinstabilityandnearlyuniversalinactivationofthep53pathway.Inthis project,wewillinvestigatearoleforMre11-mediatedtumorsuppressioninmurinemodelsofTNBCinitiatedby c-Mycoverexpression,Rb1deletion,and/orp53deficiency.InAim1,wewillanalyzetheeffectofMre11 deficiencyoncellcyclecheckpointactivationinresponsetooncogene-inducedDNAdamage.Timelapse microscopyofDNAdamageandcellcyclestatetransitionswillbeperformedinpreneoplasticprimary mammaryepithelialcells.InAim2,wewillusesinglecellwholegenomesequencingtoquantifystochastic chromosomalaberrationsthataccumulateduringoncogene-inducedreplicationstressinprimarymammary epithelialcells.TheeffectofMre11and/orp53deficiencyonthechromosomalinstabilityphenotypeatdifferent stagesoftumorigenesiswillbeanalyzed.InAim3,wewillengineerMre11mutationsinaRb1/p53-deficient murineTNBCmodeltomeasureeffectsontumorlatency.Geneexpressionandgenomicscarsignatures associatedwithMre11deficientbreastcancerswillbedetermined,andcomparedtosignaturesofBrca1 deficiency.ApanelofpharmacologicalDDRpathwayinhibitorswillbetestedtoidentifytargetablesynthetic lethalvulnerabilitiesofMre11deficiency.Collectively,thisprojectwillprovideinsightintop53-independent mechanismsofMre11-mediatedtumorsuppression,andidentifymolecularsignaturesandtherapeutic susceptibilitiesofMre11deficientbreastcancer.